1. Field of the Invention
The present invention relates to a vehicle driving environment recognition apparatus that is capable of precisely capturing driving environment in front of a vehicle with an onboard camera, when the vehicle runs at night.
2. Description of Related Art
There has conventionally been known a technique of capturing a surrounding driving environment by an onboard camera, detecting an three-dimensional object such as a pedestrian, a vehicle ahead, or a traffic light, or a lighting color of the traffic light, from the captured driving environment, and utilizing the detected result for an adaptive cruise control (ACC) or a collision prevention control (pre-crash control).
In this case, when a vehicle runs at night, a recognition target such as a road sign, a white line and pedestrian cannot be recognized by a reflection of sunlight unlike the case when the vehicle runs in the daytime. Therefore, the onboard camera recognizes a recognition target by a reflection light of an irradiation from a headlight of the vehicle, or detects a light source such as a lighting taillight of a vehicle ahead, an oncoming headlight, or a traffic light, in order to detect a distance between the vehicle and the recognition target or the light source. For this, there has also been a technique of executing a light distribution control in which a headlight is generally set as high beam, and changed to low beam when the onboard camera recognizes an oncoming vehicle.
An exposure amount is set to be large in most cases, since an amount of light incident on the onboard camera is decreased during a drive at night. However, when the exposure amount of the onboard camera is set to be large, a light quantity from a light source, such as a lighting headlight of an oncoming vehicle, or a traffic light, is too strong, and thus a blooming might occur. On the other hand, when the exposure amount is set to be smaller, it is difficult to recognize a recognition target that does not emit light.
Japanese Unexamined Patent Application Publication No. 2005-92857 describes a technique to solve this problem. The technique analyzes an image that is captured by changing an exposure amount to a high-luminance detection exposure amount and to a low-luminance detection exposure amount, so as to detect a light source such as a taillight of a vehicle running far ahead, or a lighting headlight of an approaching oncoming vehicle. The technique then obtains a width of the vehicle of interest from the distance between left and right light sources (headlight or taillight) displayed on the imaging surface of the onboard camera, thereby obtaining the distance between the vehicle ahead or oncoming vehicle and the vehicle based upon the width of the vehicle on the image and a focal distance of the onboard camera.
In the technique described in the above-mentioned publication, the distance between the left and right headlights or the left and right taillights, displayed on the imaging surface of the camera has to be calculated first in order to obtain the distance between the vehicle ahead or the oncoming vehicle and the vehicle.
However, since the light sources look on the image look different between the case of using the high-luminance detection exposure amount and the case of using the low-luminance detection exposure amount, it is difficult to detect the center of the headlights or the center of the taillights. Therefore, the distance between the left and right headlights or the left and right taillights cannot precisely be detected. Accordingly, the distance between the vehicle ahead or the oncoming vehicle and the vehicle cannot precisely be obtained at all times.
In the technique described in the above-mentioned publication, the onboard camera always makes a changeover between the high-luminance detection exposure amount and the low-luminance detection exposure amount, regardless of the light distribution (high beam and low beam) of the headlight, and regardless of whether or not there is a vehicle ahead or oncoming vehicle. Therefore, the high-luminance detection exposure amount can be employed even under the environment where higher precision detection can be attained by using only the low-luminance detection exposure amount, for example. Accordingly, the precision of recognizing the driving environment at night is reduced, and further, it becomes difficult to precisely detect the distance between the recognition target and the vehicle.